Cationically charged water soluble or water dispersible polymers are utilized in a variety of processes that involve the separation of solids or immiscible liquids which are dispersed or suspended in water from water, and the subsequent dewatering of solids containing water. These types of polymers, which may be natural or synthetic, are broadly termed coagulants and flocculants.
Cationically charged polymers neutralize the anionic charge of the suspended solids or liquids which are to be removed. These solids or liquids may be waste which must be removed from water. Alternatively, the solids may be desirable products which are recovered from aqueous systems, such as coal fines, which can be coagulated or flocculated and thereafter sold as fuel.
When it is desirable to remove suspended solids, which are suspensions of organic matter of a proteinaceous or cellulosic nature, a variety of processes may be utilized, including sedimentation, straining, flotation, filtration, coagulation, flocculation and emulsion breaking. Additionally, after suspended solids are removed from the water, they must often be dewatered so that they may be further treated or properly disposed of. Liquids treated for solids removal often have from as little as several parts per billion of suspended solids or dispersed oils, to very large amounts of suspended solids or oils. Solids being dewatered, for example sludges obtained from the biological degradation of wastewater, may contain anywhere from 0.25 weight percent solids, up to about 40 or 50 weight percent solids material. Liquid/solids separation processes are designed to remove solids from water, or liquids from solids.
While strictly mechanical means have been used to effect solids/liquid separation, modern methods often rely on mechanical separation techniques which are augmented by synthetic and natural cationic polymeric materials to accelerate the rate at which solids can be removed from water. These processes include the treatment of raw water with cationic coagulant polymers which settle suspended inorganic particulates and make the water usable for industrial or municipal purposes. Other processes requiring solids/liquid separation include the removal of colored soluble species from paper mill effluent wastes, the use of organic flocculant polymers to flocculate industrial and municipal waste materials, sludge recovery and emulsion breaking.
Regarding the mechanism of separation processes, particles in nature have either a cationic or anionic charge. Accordingly, these particles often are removed by a water soluble coagulant or flocculant polymer having a charge opposite to that of the particles to be removed. This is referred to as a polyelectrolyte enhanced liquid/solids separation process, wherein a water soluble or dispersible ionically charged polymer is added to neutralize the charged particles or emulsion droplets to be separated.
The dewatering of sewage sludges and similar organic suspensions, may be augmented by mixing into them chemical reagents in order to induce a state of coagulation or flocculation which thereby facilitates the process of separation of water. For this purpose, lime or salts of iron or aluminum have been utilized. More recently, synthetic polyelectrolytes, particularly certain cationic copolymers of acrylamide have been found to be useful.
Exemplary of cationic polymers for dewatering is U.S. Pat. No. 3,409,546 which describes the use of N-(amino methyl)-polyacrylamides in conjunction with other cationic polymers for the treatment of sewage sludges. U.S. Pat. No. 3,414,514 describes the use of a copolymer of acrylamide and a quaternized cationic methacrylate ester. Utilization of polyethyleneimines and homopolymers of cationic acrylates and methacrylates and other cationic polymers such as polyvinyl pyridines is also known. Another class of cationic polymers used to dewater sludges is described in U.S. Pat. No. 3,897,333.
Another example of a cationic polymer useful for sludge treatment is U.S. Pat. No. 4,191,645, in which cationic copolymers prepared from a nonionic monomer, such as acrylamide, and a cationic monomer, such as trimethylammoniumethylmethacrylate methyl sulfate quaternary (TMAEM.MSQ) or dimethylarninoethylacrylate methyl sulfate quaternary (DMAEM.MSQ) are disclosed. Further examples of polymeric treatments for sludge dewatering include the 1,4-dichloro-2-butene dimethylamine ionene chloride polymer disclosed in U.S. Pat. No. 3,928,448 and the block copolymers disclosed in U.S. Pat. No. 5,234,604.
Notwithstanding the variety of commercially available polymers which have been found to be capable of flocculating or coagulating organic sludges, there are various circumstances which tend to limit the usefulness of these reagents. Thus, while for certain sludges economical treatments with these known reagents are feasible, more often sludges require very high and cost-ineffective dosages of reagents in order to treat them successfully. Moreover, variations often occur in sludge from any one source. For example, variations in the supply of material to the sludge making process and/or in the oxidizing conditions that may be involved in the production of the sludge lead to a variety of particle types which must be removed. Furthermore, it is not uncommon to encounter sludges which are, for some reason, not amenable to flocculation by any of the known polymeric flocculating agents. It is therefore an object of the invention to provide to the art a superior method for the dewatering of sludge-containing industrial waste waters.